linux-kernel - Re: [PATCH v5 6/6] mm: handle userfaults under VMA lock

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Thu, 29 Jun 2023 12:32:56 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     akpm@...ux-foundation.org, willy@...radead.org, hannes@...xchg.org,
        mhocko@...e.com, josef@...icpanda.com, jack@...e.cz,
        ldufour@...ux.ibm.com, laurent.dufour@...ibm.com,
        michel@...pinasse.org, liam.howlett@...cle.com, jglisse@...gle.com,
        vbabka@...e.cz, minchan@...gle.com, dave@...olabs.net,
        punit.agrawal@...edance.com, lstoakes@...il.com, hdanton@...a.com,
        apopple@...dia.com, ying.huang@...el.com, david@...hat.com,
        yuzhao@...gle.com, dhowells@...hat.com, hughd@...gle.com,
        viro@...iv.linux.org.uk, brauner@...nel.org,
        pasha.tatashin@...een.com, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH v5 6/6] mm: handle userfaults under VMA lock

On Wed, Jun 28, 2023 at 05:19:31PM -0700, Suren Baghdasaryan wrote:
> On Wed, Jun 28, 2023 at 10:32 AM Peter Xu <peterx@...hat.com> wrote:
> >
> > On Wed, Jun 28, 2023 at 10:25:29AM -0700, Suren Baghdasaryan wrote:
> > > Enable handle_userfault to operate under VMA lock by releasing VMA lock
> > > instead of mmap_lock and retrying. Note that FAULT_FLAG_RETRY_NOWAIT
> > > should never be used when handling faults under per-VMA lock protection
> > > because that would break the assumption that lock is dropped on retry.
> > >
> > > Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> >
> > Maybe the sanitize_fault_flags() changes suite more in patch 3, but not a
> > big deal I guess.
> 
> IIUC FAULT_FLAG_RETRY_NOWAIT comes into play in this patchset only in
> the context of uffds, therefore that check seems to be needed when we
> enable per-VMA lock uffd support, which is this patch. Does that make
> sense?

I don't see why uffd is special in this regard, as e.g. swap also checks
NOWAIT when folio_lock_or_retry() so I assume it's also used there.

IMHO the "NOWAIT should never apply with VMA_LOCK so far" assumption starts
from patch 3 where it conditionally releases the vma lock when
!(RETRY|COMPLETE); that is the real place where it can start to go wrong if
anyone breaks the assumption.

Thanks,

-- 
Peter Xu